output/osx: improve sample rate selection

The formula in osx_output_score_sample_rate() to detect multiples of
the source sample rate was broken: when given a 44.1 kHz input file,
it preferred 16 kHz over 48 kHz, because its `frac_portion(16)=0.75`
is smaller than `frac_portion(48)=0.91`.

That formula, introduced by commit 40a1ebee29, looks completely
wrong.  It doesn't do what the code comment pretends it does.

Instead of using that `frac_portion` to calculate a score, this patch
adds to the score only if `frac_portion` is nearly `0` or `1`.  This
means that the factor is nearly integer.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/904

NEWS

@@ -4,6 +4,8 @@ ver 0.21.25 (not yet released)
 * input
   - file: detect premature end of file
   - smbclient: don't send credentials to MPD clients
+* output
+  - osx: improve sample rate selection
 * Windows/Android:
   - fix Boost detection after breaking change in Meson 0.54
 

src/output/plugins/OSXOutputPlugin.cxx

@@ -342,7 +342,8 @@ osx_output_score_sample_rate(Float64 destination_rate, unsigned source_rate)
 	double int_portion;
 	double frac_portion = modf(source_rate / destination_rate, &int_portion);
 	// prefer sample rates that are multiples of the source sample rate
-	score += (1 - frac_portion) * 1000;
+	if (frac_portion < 0.01 || frac_portion >= 0.99)
+		score += 1000;
 	// prefer exact matches over other multiples
 	score += (int_portion == 1.0) ? 500 : 0;
 	if (source_rate == destination_rate)